Inkjet recording apparatuses are known as a type of image recording apparatuses used in, for example, printers, facsimile machines and photocopiers. Such inkjet recording apparatus records images by ejecting ink on a recording medium. Because of its uncomplicated image forming process, the inkjet recording apparatus itself can be easily simplified. Moreover, the image forming process of the inkjet recording apparatus dose not require heating the recording medium, and thus it can save energy consumption, and thus it is desirable for the environment. It also has many advantages such as capability of quick recording of high-resolution images, low running cost, low noise level and capability of using a variety of color inks for recording images having a wide color gamut.
Commonly used inks for the inkjet recording apparatus are water-based inks containing a colorant, wetting agents such as glycerine, penetrating agents for controlling permeability of ink to recording paper, surfactants and other additives.
In the inkjet recording method, images are recorded on a recording medium from such liquid water-based ink absorbed on the recording medium. For this mechanism of forming images, variety of inkjet paper having better ink absorbency and capability of fixing and protecting coloring components has been developed. In terms of cost, however, as inkjet paper is produced by applying multiple coatings on a paper base, its production cost is high. And further, due to the large amount of chemical agents contained inkjet paper, it has a poor recyclability compared with regular paper. Thus, inkjet inks that can record high quality images not only on inkjet paper but also on regular paper have been desired.
The obstacle to forming high quality images on regular paper is that is has a poorer ink absorbency than inkjet paper and it dose not provide surface coatings suitable to accept inkjet ink. Thus, it is important to solve the following problems to achieve high quality images on regular paper, the problems include (1) occurrence of feathering (2) occurrence of bleeding (3) reduction in image density (4) degradation of color gamut (5) degradation of water resistance (6) degradation of light resistance (7) degradation of gas resistance (8) degradation of ink fixation characteristic, and (9) passing through to the other side of a recording medium.
In recent years, pigments having better dispersibility and smaller particle diameters have been developed, and such pigments have been started to be used in many inkjet inks. In addition to conventional approaches (including adding surfactants and/or water-soluble resins) to improve the dispersibility of pigments, methods of surface modification, including oxidation treatment on the surface of pigment particles, sulfonation treatment and graft polymerization, have been tried for obtaining hydrophilicity in the pigments to thereby improve dispersion stability thereof. Such pigments can solve (5), (6) and (7) of the above mentioned problems, however, there still exist problems in reliability of inks using the pigments that, because of poor density and color gamut of the pigments compared with dyes, the inks have poorer ejection stability, long term storage stability and redispersability than inks using dyes. Thus, the reliability, density and color gamut of the inks using the pigments have been important problems to be solved. In order to solve those problems, many techniques using colored polymer particles, particularly using emulsioned particles of a polyester or vinyl polymer in inks for inkjet recording have been proposed. Examples of such inks include those disclosed in Patent Literature 1 and non-Patent Literature 1. Examples of such inks further include those containing dispersed compositions of a water-insoluble resin which internally contains a colorant. In addition, it is commonly known that conventional inks using organic pigments as colorants can provide better image density and better color reproducibility on regular paper than conventional pigment inks using water-soluble dispersants.
Conventionally, techniques to obtain suitable permeability to paper in inks have been studied to thereby prevent ink from causing feathering and bleeding and from passing through to the other side of a recording medium, and to improve image density and color gamut. For example, a commercial available ink for inkjet recording has a surface tension of lower than 35 mN/m, and thereby the ink has a high permeability to paper. Such high permeable inks can effectively prevent the occurrences of bleeding and have a good drying characteristic on regular paper. However, the inks tend to cause feathering and record images with insufficient density and images/characters with vagueness.
On the other hand, some commercial available inks have a surface tension of higher than 35 mN/m, and thereby the inks have a slow permeability to paper. Such slow permeable ink slowly permeates into paper to enable it to be accumulated on the surface area of paper. Thus, it can effectively prevent the occurrence of feathering, improve image density and color gamut and prevent it from passing through to the other side of paper. The disadvantages of such low permeable ink are that, because of its low permeability, the drying characteristic drastically degrades on regular paper, and it results in degradation of ink fixation characteristics and causing bleeding on multi-colored images.
Thus, an ink-set which combines a low permeable ink with a high permeable ink has been studied. Such technique can prevent inter-color bleeding, and thus it enables to record high quality images. However, when such ink is used in two-sided printing, it brings low productivity because it requires a long drying time. Thus, some commercial available recording apparatuses are equipped with a heater to reduce the drying time of ink. In such apparatuses, paper is heated with the heater before and after ink is placed thereon. Examples of such apparatuses include the one disclosed in Patent Literature 2. Such disclosed apparatus, however, tends to be large and complicated as it contains a heater. And further, it consumes great amount of energy for heating paper. Thus, the apparatus has no advantages which an inkjet recording apparatus supposed to have.
In addition to the above-stated inks for inkjet recording, many attempts have been made to develop inks which not only have reliability and but also can record high quality images. For example, many inks having as low a viscosity as possible have been developed to prevent them from blocking the nozzles of the inkjet head. Patent Literature 3 proposes an ink that prevents the aggregation of pigment particles in order to prevent the ink from being spread, and can further prevent it from causing the generation of blank spots in images. Patent Literature 3 suggests such ink can be obtained by adjusting the change in its viscosity within 10 times higher than initial viscosity and particle diameters within 3 times larger than initial diameters when the ink is concentrated 2 times. With this technique, however, it is difficult to record high quality images on regular paper with the proposed ink.
In the ink disclosed in Patent Literature 4, the remnant of the ink after its volatile compositions has evaporated is liquid, and the viscosity of the ink in that state is within 10 times higher than the initial viscosity. The disclosed ink has an excellent reliability, while the ink itself is a dye ink which is disadvantageous in terms of image quality compared with a pigment ink.
Patent Literature 5 discloses an ink whose viscosity increases within 600 times after water evaporates therefrom under 60° C. The disclosed ink balances its durability with both reliability and image quality of the ink by adding a water-soluble polymer, while the ink itself is also a dye ink, and thus has a poor water resistance.
Patent Literature 6 suggests that an ink that can record high quality images requires its viscosity be adjusted in the range of from 5 mPa·s to 15 mPa·s. The disclosed technique suggests adjusting the initial evaporation rate of the ink to improve its reliability, and further suggests adding a certain sort of compounds as viscosity adjusters to obtain a desired viscosity in the ink. The disclosed technique thus can solve (3) and (4) of the above-stated problems. The problem of the disclosed technique is that the uniformity in the particle diameter of the used pigment is not taken into consideration. Thus, although it states that the ink has an excellent reliability after left 24 hours, the ink may not be reliable enough after left longer period of time, depending on the configuration of a head and the diameter of nozzles for ejecting ink drops.
As described above, although it is necessary that inks have a high viscosity for rapidly forming high quality images, it is difficult to obtain reliability on such high viscosity inks.
Patent Literatures 7 and 8 each propose adding a water-insoluble resin into a pigment ink to achieve better image quality. Patent Literature 9 proposes an ink that can improve the quality of images formed on regular paper. The disclosed ink has the ratio of a pigment to a resin emulsion in the range of 1:0.1 to 1:1 and an average diameter of a coloring component in the range of 0.3 μm to 1.2 μm. Such inks containing the resin emulsion can prevent the occurrences of bleeding and feathering, while it cannot provide a sufficient image density and reliability as an ink for inkjet recording.
Patent Literature 10 proposes an ink containing a water insoluble resin and having its minimum film formation temperature at 40° C. or higher. Patent Literature 11 proposes an ink having an emulsion diameter of 50 nm or smaller. In those proposed techniques, however, the reliability and image quality are still insufficient and yet to be improved.
Patent Literature 12 discloses an ink containing a pigment, saccharides or derivatives thereof, a polyol having 5 or more hydroxyl groups and a resin emulsion. It states that the ink can record clearer images/characters. But the ink has a low permeability to regular paper, and thus has problems in, for example, preventing inter-color feathering, bleeding and degradation in ink fixation characteristic and ink drying time.
Patent Literature 13 proposes an ink having solid contents of a water dispersible resin and a self-dispersible pigment in the range of 1.0% by mass to 16% by mass. It states that the ink can provide better image quality on regular paper. Although the disclosed ink can provide better water resistance than dye inks, its water resistance is still insufficient to be used with maker pens which are commonly used on regular paper.
Adding a dispersed pigment and a resin emulsion into inks increases the solid content thereof and thus increases the viscosity of the ink. This method to increase ink viscosity has been studied as a means to improve characteristics of ink. Patent Literatures 12 and 14 propose inks in which resin forming micelle-aggregate is added to increase the solid content thereof, while changes in the change in the viscosity of the inks is kept at a small increase. The proposed inks still have an insufficient permeability to regular paper, and thus the quality of images recorded with the ink is still to be improved. The permeability to paper is explained by capillarity represented by Lucas-Washburn equation. Higher viscosity of an ink requires the ink have a lower surface tension and the contact angle of ink drops to paper be arranged in order to obtain sufficient permeability of the ink.
Patent Literatures 15 and 16 propose using polyalcohol alkylether in an ink with a high solid content to increase its permeability. In the proposed inks, however, the viscosity increases accompanied by increase in the solid content, and thus sufficient permeability may not be obtained in the inks, resulting in the degradation of image quality.
That even small amounts of silicone surfactants and/or fluorinated surfactants can increase the permeability of an ink is known, and many attempts have been made to apply this means. For example, Patent Literatures 17 and 18 each propose an inkjet ink containing a fluorinated surfactant. And further, Patent Literatures 19 and 20 each propose an ink having a dispersed pigment and a fluorinated surfactant. Patent Literature 21 proposes an ink which contains a fluorinated surfactant and polymer particulates containing either a water-insoluble and/or hardly-soluble coloring material, and has polymer emulsion and a viscosity of 5 mPa·s or more.
However, color phase changes with increase of the content of the solidified pigments in those proposed inks. Thus a high concentration pigment will not provide desirable color phase, while a self-dispersible pigment will not provide sufficient water resistance and ink fixation characteristic.
[Patent Literature 1]: Japanese Patent Application Laid-Open (JP-A) No. 2000-191972
[Patent Literature 2]: JP-A No. 55-69464
[Patent Literature 3]: JP-A No. 2002-337449
[Patent Literature 4]: JP-A No. 2000-095983
[Patent Literature 5]: JP-A No. 09-111166
[Patent Literature 6]: JP-A No. 2001-262025
[Patent Literature 7]: JP-A No. 55-157668
[Patent Literature 8]: Japanese Patent Application Publication (JP-B) No. 62-1426
[Patent Literature 9]: JP-A No. 04-332774
[Patent Literature 10]: Japanese Patent (JP-B) No. 2867491
[Patent Literature 11]: JP-A No. 04-18462
[Patent Literature 12]: JP-B No. 3088588
[Patent Literature 13]: JP-A No. 2004-35718
[Patent Literature 14]: JP-A No. 2004-99800
[Patent Literature 15]: JP-A No. 2004-155867
[Patent Literature 16]: JP-A No. 2004-203903
[Patent Literature 17]: JP-B No. 2675001
[Patent Literature 18]: JP-B No. 2667401
[Patent Literature 19]: JP-A No. 04-211478
[Patent Literature 20]: JP-A No. 2003-277658
[Patent Literature 21]: JP-A No. 2003-226827
[Non-Patent Literature 1]: “Kinousei ganryou no gijutu to ouyou” published by CMC Publishing CO., LTD.